Most earthquakes occur at Plate Boundaries The deepest earthquakes occur at subduction boundaries.
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Transcript of Post Falls High School, Physical Geology. PLATES BOUNDARIES Earthquakes occur along plate boundaries...
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Post Falls High School, Physical Geology
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PLATES
BOUNDARIES
Earthquakes
occur along plate
boundaries
SEISMIC
ACTIVITY
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Stars indicate earthquake foci.A. Divergent –They are
Shallow and weak.B. Subduction – Deep
(down to 700 km) and very strong.
C. Convergent – Down to 300 km and sometimes strong.
D. Transform Fault – Down to 100 km and often strong.
EARTHQUAKES INPLATE MARGINS
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An Earthquake is caused by sudden release
of elastic energy stored in rocks.
Elastic deformation: Any change
of shape or size that disappears
when the deforming forces are
removed.
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The focus of an earthquake is the site of first
movement on a fault and the center of energy
release. The epicenter of an earthquake is the
point of the Earth's surface that lies vertically
above the focus.
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When an earthquakeoccurs, the elasticallystored energy iscarried outward fromthe focus to otherparts of the Earth byvibrations. Thesevibrations are calledseismic waves andspread outspherically in alldirections, just assound waves do.
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Body Waves: Travel outward in all directions
from the focus and have the capacity to travel
through the Earth's interior.
P (primary) waves -compression/expansion
S (secondary) waves - shear
Surface Waves: Travel around but not through
the Earth; they are guided by the Earth's surface.
SEISMIC WAVES
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BODY WAVES
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Body WaveP waves in motion
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Body WaveS wave in motion
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Surface WaveL wave in motion
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TRAVEL
TIME
OF
SEISMIC
WAVES
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REFRACTION
AND
REFLECTION
OF BODY
WAVES
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TRAVEL PATHS OF DIRECT AND REFRACTED BODY WAVES
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P waves are the fastest waveS waves are slightly slowerL waves are the slowest of the three waves
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A highly simplified simulated recording of earthquake
waves (a seismogram). This seismogram is a simulation.
The actual records of earthquake waves are far more
complicated than what is presented here
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The speed differences between waves are used to determine the distance away an earthquake is from the recording station.
A seismogram can not tell in what direction the earthquake occurred.
Seismologist use simple triangulation (math) to determine the epicenter of an earthquake.
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LOCATING AN EPICENTER
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The Richter magnitude scale is used to measure the strength of an earthquake. It assigns a single number to quantify the size of an earthquake. It is a base-10 logarithmic scale.
Developed in 1935 by Charles Richter in collaboration with Beno Gutenberg.
Measuring Earthquake Magnitude
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Magnitudes Effects
Less than 3.5 Generally not felt, but recorded
3.5-5.4 Often felt, but rarely causes damage
Under 6.0 At most slight damage to well-designed buildings. Can cause major damage to poorly constructed buildings over small regions.
6.1-6.9 Can be destructive in areas up to about 100 kilometers across where people live
7.0-7.9 Major earthquake. Can cause serious damage over larger areas
8 or greater Great earthquake. Can cause serious damage in areas several hundred kilometers across
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The Modified Mercalli Intensity Scale is commonly used in the United States by seismologists seeking information on the severity of earthquake effects. Intensity ratings are expressed as Roman numerals between I at the low end and XII at the high end.
The Intensity Scale differs from the Richter Scale in that the effects of any one earthquake vary greatly from place to place, so there may be many Intensity values (e.g.: IV, VII) measured from one earthquake
Measuring Earthquake Magnitude
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I. People do not feel any Earth movement.
II. A few people might notice movement if they are at rest and/or on the upper floors of tall buildings.
III. Many people indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an earthquake is occurring.
IV. Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. The earthquake feels like a heavy truck hitting the walls. A few people outdoors may feel movement. Parked cars rock.
V. Almost everyone feels movement. Sleeping people are awakened. Doors swing open or close. Dishes are broken. Pictures on the wall move. Small objects move or are turned over. Trees might shake. Liquids might spill out of open containers.
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VII Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage. People have difficulty standing. Drivers feel their cars shaking. Some furniture breaks. Loose bricks fall from buildings. Damage is slight to moderate in well- built buildings; considerable in poorly built buildings.
VIII. Drivers have trouble steering. Houses that are not bolted down might shift on their foundations. Tall structures such as towers and chimneys might twist and fall. Well-built buildings suffer slight damage. Poorly built structures suffer severe damage. Tree branches break. Hillsides might crack if the ground is wet. Water levels in wells might change.
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IX. Well-built buildings suffer considerable damage. Houses that are not bolted down move off their foundations. Some underground pipes are broken. The ground cracks. Reservoirs suffer serious damage.
X. Most buildings and their foundations are destroyed. Some bridges are destroyed. Dams are seriously damaged. Large landslides occur. Water is thrown on the banks of canals, rivers, lakes. The ground cracks in large areas. Railroad tracks are bent slightly.
XI. Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground. Underground pipelines are destroyed. Railroad tracks are badly bent.
XII. Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move.
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Moment Magnitude
• Richter's original methodology is no longer used because it does not give reliable results when applied to M >= 7 earthquakes and it was not designed to use data from earthquakes recorded at epicentral distances greater than about 600 km. It is, therefore, useful to separate the method and the scale in releasing estimates of magnitude to the public
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Seismic moment of an earthquake is given by
The actual moment magnitude is
calculate using
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Comparison between the Richter and Moment Magnitude Scales
Earthquake Richter Scale Moment Magnitude
New Madrid, MO, 1812 8.7 8.1
San Francisco, CA 1906 8.3 7.7
Prince William, AK 1964 8.4 9.2
Northridge, CA,1994 6.4 6.7
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Earthquake hazards exist
throughout the United States
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GREAT U.S. EARTHQUAKES
• New Madrid 1811 and 1812 Earthquakes
• The Great 1906 San Francisco Earthquake -
April 18, 1906 5:12 AM
• The great Alaska earthquake of 1964 – March
27,1964 5:36 p.m.
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Although most Americans associate
earthquakes with California, the tremors
that shook the Mississippi valley in
southeast Missouri from December 16,
1811, through February 7, 1812, are
among the most violent quakes ever to hit
the North American continent in recorded
history. Collectively known as the New
Madrid earthquakes, these quakes affected
more than 1 million square miles.
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1811-1812 New Madrid Earthquakes
Roman numerals indicate estimated Modified Mercalli intensities for a 6.5 magnitude earthquake.
The New Madrid seismic zone is so named because
the town of New Madrid, Missouri was the closest
settlement to the epicenters of the 1811-1812 quakes.
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Isoseismal map
for the earthquake
of December
16,1811 (first of
the 1811-1812
New Madrid
series). Felt in
Washington &
Boston (causing
church bells to
ring).
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Severe shaking accompanied the powerful New Madrid earthquakes. By winter's end, few houses within 250 miles of the Mississippi River town of New Madrid (Missouri) remained undamaged.
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“On the morning of Monday, the 16th of December, 1811,
several shocks of earthquakes were felt at the city of
Washington. The first of these happened at three o'clock;
and in some houses was considerable enough to shake the
doors and windows, and wake persons from their sleep.
There were successive tremors. Tassels of curtains were
seen to move; and pitchers of washing-stands were heard to
rattle upon their basins. The sound was very distinguishable,
and was believed by many to pass from southwest to
northeast. The alarm was so great in some families, that
searches were made from room to room, to discover the
robbers who were imagined to have broken into the houses.”
A Detailed Narrative of the Earthquakes which occurred on the 16th day of December, 1811 Samuel L. Mitchill
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The Great 1906 San Francisco Earthquake
The quake lasted only a minute but caused
the worst natural disaster in the nation's
history. Modern analysis estimates it
registered 8.25 on the Richter scale. The
greatest destruction came from the fires the
quake ignited. These ravaged the city for three
days before burning themselves out.
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This photograph by Arnold Genthe shows
Sacramento Street and approaching fire.
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The Great fire as seen from a ferry boat in the bay
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Refugees search for souvenirs on sidewalk in front of the Stanford mansion on Powell Street at California.
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This photograph, taken from a tethered balloon five weeks after the great earthquake
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The quake awoke G.A. Raymond as he slept in his room at the Palace Hotel. This is an excerpt from his description of
escape.
“How I reached the ferry I cannot say. It was
bedlam, pandemonium and hell rolled into one.
There must have been 10,000 people trying to
get on that boat. Men and women fought like
wildcats to push their way aboard.”
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16 Views of the 1906 16 Views of the 1906 San FranciscoSan Francisco
Earthquake and FireEarthquake and Fire
From the Archives of From the Archives of The Museum of the City of San FranciscoThe Museum of the City of San Francisco
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The Great Fire as seen from a ferry boat in the Bay.
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Fire around the Call Building at Third and MarketStreets. Emporium, at Powell and Market, is at right.
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“Ham and Egg” fire burns at Grove and Laguna streets.View is along Grove Street. City Hall dome can be seen in the smoke cloud.
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Close-up view of refugees fleeing along Grove Street.The large clusters of refugees along the street suggest they are involved in rescuing trapped persons.
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Crowds gather at Market and Laguna streets to flee the Great Fire. Building at lower center right still survives as do along Laguna. Almost all others pictured here burned.
Crowds gather at Market and Laguna streets to flee the Great Fire. Building at lower center right still survives as do along Laguna. Almost all others pictured here burned.
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View from Laguna and Market streets of the Great Fireburning through the Mission District.
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View of the Great Fire as seen from Chinatown.
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Dreadful earthquake damage, along Grant Ave., in Chinatown.
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Refugees search for souvenirs on sidewalk in front of the Stanford mansion on Powell Street at California.
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Sunken cobblestone paving along Spear at Market Street.
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Ruined homes and pavement on Dore Street near Folsom.
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Mission Dolores on Dolores at 16th street, and thewrecked Parish church next to it.
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Damaged warehouse in the area of Nineteenth andFolsom streets.
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Earthquake-wrecked water main near Crystal Springs Reservoir in San Mateo County.
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Broken water mains on Valencia between 18th and 19th streets.
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Refugees built shanties from any available materials to house themselves after the disaster. This encampment is near the Marina. The Army later provided tents for refugees.
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San Francisco earthquake
1989
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The Great Alaska Earthquake of 1964
The Alaskan earthquake of 1964 was the
largest earthquake in North America and the
second largest ever recorded (largest occurred
in Chile in 1960). The strong ground motion
reported in the Anchorage area lasted about 4-
5 minutes which triggered many avalanches,
landslides and tsunamis.
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The Great Alaska Earthquake of 1964
The 1964 earthquake caused 115 deaths in
Alaska alone, with 106 of these due to
tsunamis which were generated by tectonic
uplift of the sea floor, and by localized
subareal and submarine landslides.
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The Great Alaska Earthquake of 1964
Tsunamis generated by the 1964 earthquake
(and their subsequent damage, loss of life,
etc.) were recorded throughout the Pacific.
This was the most disastrous tsunami to hit
the U.S. West Coast and British Columbia in
Canada. The largest wave height for this
tsunami was reported at Shoup Bay, Valdez
Inlet (67 meters)
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PHOTOGRAPHS OF DAMAGE ASSOCIATED WITH THE 1964 ALASKA EARTHQUAKE
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Government Hill Elementary School in Anchorage was one of many schools severely damaged by quake-triggered landslides.
These thick trees near Port Valdez splintered into pieces against the force of massive tsunami waves.
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In Idaho, too!
• October 28, 1983• Mt. Borah/Challis• 7.3 on the Richter
Scale
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Eyewitness Account
“On the morning of October 28, Don Hendrickson and John Turner were on a dirt road in Arentson Gulch, Idaho, in a 4WD pickup, looking for elk, when Don, after feeling light-headed and dizzy, saw the roadfall away in front of his vehicle, as if a sinkhole had formed. This was followed by the formation of a surface rupture about 20m in front of the vehicle, with the only sound being the crumbling of earth in front of them. This was followed by violent shaking and a deafening roar, the entire episode lasting 10-15 seconds.”
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Eyewitness Account
“At the same time, Mrs Lawana Knox was seated on a north slope of a hill looking for her husband, a hunter, when a 1 to 1.5m high fault scarp formed in front of her at about 300m in distance, reaching its full height in about one second! The scarp seemed to tear from NW to SE along the flank of the mountain just as though one took a paint brush and painted a line along the hill. It took only seconds to extend several miles along the range front…..”
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Secondary Effects
Most of the damage done by earthquakes is due
to their secondary effects, those not directly
caused by fault movement, but resulting instead
from the propagation of seismic waves away from
the fault rupture. Secondary effects result from
the very temporary passage of seismic waves,
but can occur over very large regions, causing
wide-spread damage.
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Secondary Effects
• Natural - Landslides/mudflow
- Liquifaction - Subsidence - Tsunamis
Man-made- Dam failures - Secondary fires
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Earthquake induced landslide - Madison River, Montana, 1959 - 80 million tons of rock released as a result of the quake- River was dammed creating Quake Lake
Landslides/mudflow
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Liquifaction Flow of material, particularly that with a high H2O content
Ejection of sand-slurry through overlying soil etc. Causes sand volcanoes, destroys crop land, disrupts underground utilities, weakens building foundations
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• Tsunamis
- small amplitude, long wavelength in
open ocean
- speed ~ 350-500 mph
- near shore the waves begin to break
and the enormous quantity of water
“piles up”
- breaking wave may be 15 - 65 m high
- Aleutian Islands, 1946; 10 hours later
hits Hawaii
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Locations of past tsunamis around the world
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The earliest description of a tsunami-type wave comes from 479 B.C. in the northern part of the Aegean Sea. Similar waves have been reported worldwide, though they are more common in the Pacific, with its earthquake-prone perimeter.
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The 2004 Indian Ocean earthquake was an undersea earthquake that occurred at 00:58:53 UTC (07:58:53 local time) on December 26, 2004. The earthquake generated a tsunami that was among the deadliest disasters in modern history.At magnitude of 9.0, it was the largest earthquake since the 9.2 magnitude Good Friday Earthquake off Alaska in 1964, and tied for fourth largest since 1900. In February 2005, new analysis suggested the magnitude was underestimated, and one study estimates it at 9.3; however, the USGS has not yet confirmed this (McKee, 9 Feb 2005)).The earthquake originated in the Indian Ocean just north of Simeulue island, off the western coast of northern Sumatra, Indonesia. The resulting tsunami devastated the shores of Indonesia, Sri Lanka, South India, Thailand and other countries with waves up to 30 m (100 feet) high It caused serious damage and deaths as far as the east coast of Africa, with the furthest recorded death due to the tsunami occuring at Port Elizabeth in South Africa, 8 000km (5 000 miles) away from the epicenter.
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Asian Tsunami, Dec., 2004
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Worldwide Fatalities
1 1887 China's Yellow River breaks its banks in Huayan Kou 900,000
2 1556 Earthquake in China's Shaanxi, Shanxi and Henan provinces
830,000
3 1970 Cyclone in Bangladesh 500,000
4 1976 Earthquake in Tangshan, China 242,000
5 2004 Asian quake disaster - toll so far 150,000
6 1923 Tokyo earthquake 140,000
7 1815 Volcanic eruption of Mount Tambora on Indonesia's Sumbawa Island
90,000
8 2003 earthquake in Bam, Iran 30,000
9 1896 Tsunami in Japan 27,000